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Share-based Compensation - Summary of RSU Activity (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2021
Restricted Stock Units (RSUs) [Member] | Long Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
The target percentage of RSUs earned and vested	105.60%

X

- Definition

The performance percentage of share-based awards earned during the period for share based compensation other than options.

+ References

No definition available.

+ Details

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Namespace Prefix:	rm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PlanNameAxis=rm_LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: